Since it must be cleaned from the muzzle, it is common for Minis to have dinged/worn rifling at the muzzle.  Carefully inspect the muzzle for damage.  Perhaps a honing or re-crowning will suffice, and be much cheaper.
Check out these links:[url]http://www.survivallink.com/howto/excs71.txt[/url], [url]http://www.iea.com/~fgrig/gun/mini14.htm[/url], [url]http://www.ruger-firearms.com/home.html[/url], [url]www.mimi14world.com[/url], [url]http://www.ruger-mini-14-firearms.com/Page1.html[/url].
Many unfairly trash these guns for poor accuracy, when only a little effort and money is needed to achieve good results.
A good barrel, good adjustable rear sights, and a well-fitting, bedded synthetic stock will fix most, if not all Mini problems.  Good luck.

Bushmaster now sells an accuracy kit for the Mini-14 which improves the magazines, simplifies the bolt - its removal and cleaning, gives you better sights, and replaces the barrel with a 16" chrome lined one that will do 1 moa all day long and makes commonly available spare parts for it. It's called an AR-15!

Seriously the Mini-14 is at least a rifle you can still buy here in California. I was doing research to replace the awful barrel on mine but passed on the O.K.M. Match barrel for $400 and the Shilen heavy barrel for $225.

Decided to mount an Aimpoint Comp M on a forward mounted scout rail instead.

My crown is in cherry shape and get 10 round groups half the size of yours. I have replaced the sights with some better ones.